1. Field of the Invention
The present invention is directed to desiccant compositions, methods for producing desiccant compositions and applications of desiccant compositions. More specifically, the present invention is directed to a composite desiccant composition having a high affinity for water wherein the desiccant includes an absorbent deposited onto a porous, high surface area support.
2. Description of Related Art
Desiccants are materials that are capable of capturing and retaining water in the form of a liquid or a vapor. Desiccants can operate by two fundamental mechanisms referred to as absorption and adsorption. Absorption occurs when a substance (e.g., water vapor) penetrates the inner structure of another (the absorbent). Absorbents for water include salts such as calcium chloride (CaCl2) and lithium chloride (LiCl). Adsorption occurs when a substance (e.g., water vapor) is attracted and held onto the surface of another (the adsorbent). Adsorbents for water vapor include highly porous hydrophilic materials such as activated carbon and silica gel.
Examples of such materials can be found in the prior art. For example, U.S. Pat. No. 4,402,717 by Izumo et al. discloses an apparatus for removing moisture and odors from air. The apparatus includes a cylindrical honeycomb structure fabricated from paper through which the moist air flows. An adsorbent such as activated carbon is incorporated into the paper during the papermaking process to deodorize the air. The paper is also impregnated with an absorbent salt such as lithium chloride by dipping the paper into a salt solution and drying.
U.S. Pat. No. 5,135,548 by Golden et al. discloses an oxygen selective desiccant. A carbon molecular sieve is impregnated with absorbent salts or inorganic oxides for the simultaneous removal of water and oxygen from air to produce nitrogen gas. The carbon molecular sieve is impregnated with the absorbent such that the volume of solvent utilized is roughly equivalent to the pore volume of the materials to be impregnated, resulting in a loading of absorbent on the carbon molecular sieve of about 5 to 10 weight percent.
Japanese Patent Publication No. JP59228935 by Hiroyasu et al. discloses a fibrous active carbon filament which is impregnated with a dehumidifying agent such a lithium chloride, lithium bromide or calcium chloride to form a dehumidifying element. The filaments are wound or woven onto a support structure to form the dehumidifying element. The amount of dehumidifying agent added to the carbon fiber is 0.5 to 90 weight percent. The agent is added by immersing the filament into a salt solution and drying the filament.
For many applications, there is a need to absorb relatively large quantities of water using only a small volume of desiccant. There is also a need for a desiccant that can absorb the water at a fast rate. For example, cooling devices that utilize a sorption cycle for cooling with water as a refrigerant require that large quantities of water vapor be removed from the system very quickly and stored in a small volume.
One aspect of the present invention is directed to a composition of matter that is a desiccant composition. The desiccant composition preferably includes a porous support material and an absorbent dispersed on the porous support material wherein the porous support material has a pore volume of at least about 0.8 cc/g and an average pore size of from about 1 to about 20 nanometers.
The porous support can include a material such as activated carbon or silica. In one embodiment, the desiccant composition includes a porous silica support material and an absorbent salt dispersed on the porous silica support.
Another aspect of the present invention is directed to a method for forming a desiccant composition, comprising the steps of providing a porous support material having a pore volume of at least about 0.8 cc/g, contacting the porous support material with a flowable medium comprising an absorbent for a time sufficient to substantially fill porosity in the porous support material and then drying the porous support material to remove liquid from the flowable medium and form a desiccant composition comprising the absorbent dispersed on the porous support. The desiccant composition preferably has a pore volume of at least about 0.4 cc/g.
According to another aspect of the present invention a method is provided for absorbing water vapor by contacting the water vapor with a desiccant composition comprising an absorbent dispersed on a porous support for a time sufficient to absorb at least about 0.4 gram of water per gram of desiccant.
The present invention is directed to desiccant compositions, methods for producing desiccant compositions and applications of desiccant compositions. The desiccant compositions of the present invention are composite desiccants that include an absorbent, such as a salt, which is dispersed onto a porous support material that has a high pore volume and a controlled pore size. The resulting desiccant has a high affinity for water and is useful in a number of applications that require the absorption of high levels of water. It is a particular advantage of the present invention that a relatively large quantity of water vapor can be absorbed in a relatively small volume of the desiccant. As used herein, the terms absorb, absorption or the like refer to the retention of the water by the desiccant, regardless of the actual mechanism by which the water is retained.
The desiccant according to the present invention includes a porous support material. The porous support material preferably has a high pore volume, and therefore a high surface area, to accommodate the absorption of large amounts of liquid by the desiccant. The pore volume, expressed as a volume per unit mass, can be measured by a nitrogen condensation method. Alternatively, the pore volume can be measured gravimetrically by wicking a wetting fluid into the pores and measuring the change in mass. Support materials having a low pore volume are not capable of forming desiccants that can accommodate the absorption of large quantities of water. For example, carbon molecular sieves, as described in U.S. Pat. No. 5,135,548 by Golden et al., have a pore volume of about 0.2 cc/g or less and are not able to retain large quantities of water.
The pore volume of the porous support according to the present invention is preferably at least about 0.8 cc/g, more preferably at least about 1 cc/g and even more preferably at least about 1.5 cc/g, such as at least about 2 cc/g. In one embodiment, the pore volume of the support material is from about 1.5 cc/g to about 2 cc/g. As is discussed above, support materials having a pore volume of less than about 0.8 cc/g will not effectively retain large amounts of water. Porous support materials having pore volumes in excess of about 8 cc/g may be useful for some applications, however the structural integrity of the support is typically too low for most applications.
In order to accommodate high levels of liquid such as water, it is also important to control the average pore size and pore size distribution of the support material. As used herein, the pore size refers to the pore diameter. The average pore size is preferably at least about 1 nanometer, such as in the range of from about 1 to about 20 nanometers. The pore size distribution is such that there are very few pores having a size of less than about 0.5 nanometers. Pore size can be measured, for example, by nitrogen adsorption.
In addition, the porous support material should be substantially non-reactive to the absorbent which is dispersed onto the porous support surface to form the desiccant. That is, the porous support material should not react with the absorbent in a manner to reduce or eliminate the water absorption properties of the absorbent.
The support material can be selected from virtually any material having the above identified properties. Preferably, the support material is hydrophilic. Particularly preferred materials include activated carbon and silica. A suitable activated carbon is available under the tradename xe2x80x9cNUCHAR BAX 1500xe2x80x9d from Westvaco Corp. A suitable silica material is available under the tradename xe2x80x9cAEROSIL TS 100xe2x80x9d from DeGussa Corp. The preferred support material will depend upon the application of the desiccant composition. For example, some support materials have a lower heat of adsorption than others. This represents the amount of heat generated when the desiccant absorbs water. This can be important where the heat generated upon adsorption of the water vapor needs to be minimized. For example, when the desiccant composition is utilized in a sorption cooling device, the heat generated by sorption of the water vapor should be minimized.
The porous support material can come in a variety of shapes and sizes selected for a particular application. In one embodiment, the porous support is provided in the form of small activated carbon pellets having a size in the range of from about 0.5 to 2 millimeters. The size of the pellets can be selected to influence the rate at which water is absorbed wherein larger pellets will absorb water at a slower rate due to the increased path length that the water vapor travels. In another embodiment, the porous support is silica having a size of from about 0.25 to 0.5 millimeters.
According to the present invention, an absorbent is dispersed onto the surface of the porous support. The absorbent is preferably an absorbent salt known in the art and can be selected, for example, from calcium chloride (CaCl2), lithium chloride (LiCl), lithium bromide (LiBr), magnesium chloride (MgCl2), calcium nitrate (Ca(NO3)2) and potassium fluoride (KF). Other absorbents can include phosphorous pentoxide (P2O5), magnesium perchlorate (Mg(ClO4)2), barium oxide (BaO), calcium oxide (CaO), calcium sulfate (CaSO4), aluminum oxide (Al2O3), calcium bromide (CaBr2), barium perchlorate (Ba(ClO4)2) and copper sulfate (CuSO4). Combinations of two or more of these salts can also be used and the particularly preferred absorbent will depend upon the application of the desiccant.
The desiccant composition according to the present invention can be formed by contacting the porous support material with a liquid solution comprising the absorbent. For example, the porous support can be contacted with a predetermined amount of liquid solution that is sufficient to fill the porosity of the porous support with the solution without substantial excess. The solution is typically aqueous-based, however non-aqueous solutions such as ethanol solutions can be used for wicking into hydrophobic porous support materials. Preferably, the solution is a saturated solution of the absorbent salt. In a preferred embodiment, the solution includes sufficient absorbent to provide a desiccant composition comprising from about 20 weight percent to about 80 weight percent of the absorbent, more preferably from about 40 weight percent to about 60 weight percent of the absorbent, and even more preferably from about 45 to about 55 weight percent of the absorbent.
Thus, the available pore volume for a given amount of porous support material is calculated and the porous support material is then contacted with a volume of the solution approximately equal to the pore volume. The entire solution wicks into the porosity, leaving virtually no excess solution filling the channels between support particles or on the outer surface.
Alternatively, the porous support can be contacted with a solution of the absorbent for a calculated amount of time so that the porosity is substantially filled without excess solution residing on the support surface.
The porous support is then dried to remove the solvent. For example, the porous support coated with the salt solution can be dried at an elevated temperature (e.g., about 200xc2x0 C.) for a sufficient period of time to drive off the solvent and crystallize the absorbent salt.
The amount of absorbent dispersed onto the porous support is carefully controlled to ensure maximum water sorption properties. For example, after impregnation of the absorbent, it is preferred that the pore volume of the desiccant is at least about 50 percent of the pore volume of the porous support, and even more preferably at least about 66 percent of the pore volume of the porous support. That is, it is preferred that if the pore volume of the porous support is about 1.5 cc/g, then the pore volume of the desiccant is preferably no less than about 0.75 cc/g, more preferably no less than about 1.0 cc/g. Such high pore volumes can be realized even at high absorbent loadings such as 40 to 60 weight percent.
The desiccant composition according to the present invention is preferably capable of absorbing at least about 100 percent of its weight in water, more preferably at least about 150 percent of its weight in water and even more preferably at least about 200 percent of its weight in water. The amount of water that can be absorbed will also be influenced by the relative humidity and temperature, as is illustrated in the examples below.
The desiccant composition of the present invention is useful in a number of applications. In many applications the desiccant is used in the form of a packed bed. The relative density of the packed bed will partially control the rate of water absorption into the bed.
The desiccant composition of the present invention can also be regenerated for multiple uses by heating the desiccant to remove water.